Until very recently, the mere mention that fluoridated water might cause adverse health effects was likely to be met with deep skepticism, even derision. The Centers for Disease Control & Prevention still calls water fluoridation the greatest health triumph of the past 50 years.

But those attitudes are beginning to change.

In March, the National Research Council (NRC) released a report saying the maximum level of fluoride the Environmental Protection Agency allows in drinking water – 4 milligrams per liter, or 4 parts per million (ppm) – harms teeth and bones. About 200,000 people in the U.S. consume water with natural fluoride levels of 4 ppm or higher. But about 65 percent of the U.S. population drinks water artificially fluoridated at a much lower level – about 1 mg per liter or 1 ppm.

Ten percent of children exposed to the maximum contaminant level of fluoride of 4 ppm develop severe dental fluorosis, a permanent condition characterized by brown stains and pitting of the teeth, the report said. Severely affected teeth are likely to decay and often must be capped.

In addition, populations with lifetime exposure to water fluoride levels of 4 ppm or higher are likely to experience more bone fractures than groups exposed to 1 ppm fluoride or less, the NRC reported.

Studies conducted in the 1940s suggested that drinking water with 1 ppm fluoride reduced tooth decay 50 to 65 percent. But by today's standards, those studies were poorly designed.

More recent research indicates that ingested fluoride plays little or no role in preventing tooth decay. But topical fluoride exposure from toothpaste probably does help. The most convincing evidence: Over the past 50 years, the reduction in tooth decay in Europe is about the same as that in the United States, yet very few Europeans drink fluoridated water. Diet improvements, such as eating more fresh food, may have played a large role in improving teeth in both continents. Fluoridated toothpaste and rinses may also provide some benefit.

Health risks of water fluoridated at 1 ppm are less definitive than those associated with 4-ppm water. But for infants, the risks are fairly straightforward. In 1997, the Institute of Medicine set a safe upper limit of 0.7 mg of fluoride a day for children under 6 months of age. Yet in 25 of the 28 largest cities of the United States (most with fluoridated water), at least 15 percent of formula- fed infants are exposed to excessive levels of fluoride from tap water used to mix the formula, according to calculations by the Environmental Working Group. For example, 61 percent of the formula-fed babies in Boston ingest too much fluoride, EWG says, based on fluoride levels published by water utilities and the average amount of formula consumed by babies.

For older children and adults, health risks still are being debated. About one third of children who live in areas with fluoridated water develop mild dental fluorosis. The teeth appear to have white patches – actually areas of incomplete mineralization.

One of the most worrisome, and possibly widespread, effects of water fluoridation is skeletal fluorosis. This is because on average, half of the fluoride ingested over a lifetime is stored in the bones.

Skeletal fluorosis has three stages: mild, moderate, and severe. The symptoms of mild and moderate skeletal fluorosis are the same as symptoms of arthritis. The only way to diagnose these stages and distinguish them from arthritis is to take a bone biopsy, an invasive procedure not usually performed. Crippling skeletal fluorosis is more easily diagnosed because victims have difficulty walking. Very few Americans develop this, but in regions of India where water has high natural fluoride levels, the disease is fairly common.

The NRC report also notes a growing body of scientific research linking fluoride to disruption of the nervous and endocrine systems, including the brain, thyroid, and pineal gland. Many Americans living in areas where the water contains 1 ppm or more of fluoride now receive doses of fluoride associated with thyroid disturbances. Even very low levels of fluoride exposure seem to suppress thyroid function.

The NRC report says: "In humans, effects on thyroid function were associated with fluoride exposure of 0.05-0.13 mg/kg/day when iodine intake was adequate, and 0.01-0.03 mg/kg/day when iodine intake was inadequate. An adult who drinks 2 liters of water, or other beverage made with fluoridated water, would receive an exposure of 2 mg per day (or 0.03 mg/kg/day for an adult weighing about 167 lb)."

NRC panel member Kathy Thiessen, a former senior scientist at Oak Ridge National Laboratory, told the Portland Tribune: "The potential is there" that water fluoridation is unhealthy. In her personal opinion, "most people should minimize their fluoride intake, which includes avoiding fluoridated water." Thyroid suppression can result in fatigue, obesity and harm to fetuses.

Consumers should consider that virtually all soft drinks, juices prepared from concentrate, and canned soups are made from fluoridated water. Also, pastas and rice – and all foods that absorb water – contain fluoride if cooked in fluoridated water. The only food that absorbs a significant amount of fluoride from the soil is tea.

For the individual, there is no easy way to remove fluoride from water. Rather than a simple charcoal or Brita filter, removal requires reverse osmosis or distillation. Most bottled waters do not contain fluoride unless it is added deliberately. However, a few brands are made from fluoridated tap water, but this is not usually noted on the label.